The present invention relates to a drive mechanism for an ignition distributor, a crank angle sensor, etc. used in the ignition system of an engine. In particular, the present invention relates to a drive mechanism using a screw gear as a power transmission system.
In the ignition distributor, the crank angle sensor, etc., the rotation of a crank shaft of the engine is transmitted to a rotating shaft of each of the ignition distributor and the crank angle sensor through a screw gear. A backlash of the screw gear of this kind tends to be structurally increased for reasons of manufacture of the screw gear.
An angular acceleration of a crank shaft or a drive shaft of the engine is generated by fluctuations of rotation caused by an explosion in cylinders, etc. The rotating shaft which is a driven shaft has a rotational inertia. Therefore, tooth faces of the drive and driven shafts come into collision with each other by the above backlash, so that an ignition timing becomes unstable and noises, abrasion and damages are involved with respect to the screw gears.
For example, Japanese Patent Examined Publication No. 51-27814 and Japanese Patent unexamined Publication No. 3-290059 (U.S. Pat. No. 5,088,459) disclose techniques for solving these problems. In these documents, a screw gear on a driven shaft comprises a main gear fixed to the driven shaft and an auxiliary gear rotatably and slidably fitted on the driven shaft. While the main and auxiliary gears are staggered from each other by an amount corresponding to backlash, a resilient spring force is applied to the auxiliary gear in a rotational direction thereof. Thus, teeth of a screw gear on the side of a drive shaft are caught by teeth of the main and auxiliary gears therebetween to remove the backlash.
In Japanese Patent Examined Publication No. 60-24311 and Japanese Patent Unexamined Publication No. 59-63367, a sliding plate adapted to rotate with a driven shaft is biassed against an end face of a housing, by resilient spring force which housing supports this driven shaft. Thus, rotational friction force is produced on the driven shaft to increase rotational torque of the driven shaft, so that self-propelling of a driven shaft system caused by its rotational moment is restrained.
Between the above measures to counter backlash, the former one is disadvantageous in that contact pressure between screw gears becomes great since a spring force (a spring force for biasing an auxiliary gear in a direction of rotation) required for removing backlash must be made great as an angular acceleration of a drive shaft and a rotational inertia of a driven shaft become great.
The latter one naturally has a limit because a spring force is made great to increase surface pressures (friction force of rotator) on contact faces, thus involving a problem such as abrasion of screw gears and parts which generate friction forces.